1. Field of the Invention
The invention relates to a firing mechanism for a gun utilized to effect the perforation of a subterranean well casing or liner having a primary actuator utilizing a fluid pressure force to detonate a first primer and a secondary or backup actuator utilizing a mechanical impact force to detonate a second primer to effect the firing of the gun in the event that the first primer fails to fire the gun.
2. Description of the Prior Art
Perforating guns have long been employed to effect the perforation of a well casing or liner in the vicinity of a production zone and to produce passages extending into such production zone. The popular perforating guns now uniformly employ the so-called "shaped charges," which are disposed in vertically and angularly spaced relationship relative to the casing axis so as to produce a large number of evenly spaced perforations with a single firing. Such shaped charges are generally ignited by a primer cord which contacts a primer end of each shaped charge container to detonate the charge contained within each such container.
The ignition of the primer cord has heretofore been generally accomplished by the production of an impact force on a detonatable primer. In some firing mechanisms, the impact force is produced by a fluid pressure actuated hammer which is driven by the fluid pressure into impact engagement with the primer. In other firing mechanisms, and particularly for perforating guns carried into the well on a tubing string, the detonation of the primer has been accomplished by the dropping of a detonating bar through the tubing string to impact upon a firing pin which in turn pierces the primer.
Regardless of the type of firing mechanism, i.e., whether fluid pressure actuated or mechanically actuated, the perforating of subterranean wells has been plagued by a failure of the primer charge to detonate and thus ignite the primer cord and effect the detonation of the shaped charges. Many well perforating operations are now performed by a perforating gun which is suspended from a packer which in turn is run into the well on a tubing string. With this arrangement, the packer may be set in the well casing at a position to locate the perforating gun adjacent to the production zone to be perforated. Once the packer is set, the casing annulus below the packer is isolated from the casing annulus above the packer. This permits the perforating operation to be performed with the fluid pressure in the casing annulus adjacent the production zone being reduced to a level below the anticipated fluid pressure of the production fluid. This operation is commonly referred to as perforating in an underbalanced condition. The primary advantage of underbalanced perforation is that it permits an immediate flow of production fluid from the passages in the perforated production zone and thus flushes out the debris which is normally associated with the perforating operation.
Since control of the fluid pressures in the vicinity of the perforating gun is essential to perform underbalanced perforating, it is only natural that a fluid pressure actuated firing mechanism for the perforating gun is a preferred mode of effecting the detonation of the shaped charges of the gun. However, even though proper pressures are applied to a hammer which is slidably and sealably mounted in a fluid pressure chamber above a detonatable primer, the application of a fluid pressure force to the hammer to drive it into impact engagement with the primer does not always result in the detonation of the primer and the firing of the perforating gun. This necessarily means that the entire tubing string, packer, firing mechanism and perforating gun must be removed from the well with the attendant substantial increase in completion time and cost.
There is, therefore, a definite need and economic justification for a backup or redundant firing system which can be mechanically actuated in the event of failure of the fluid pressure actuated firing mechanism to effect the firing of the perforating gun.